NEUROD INDUCES PHOTORECEPTOR CELL OVERPRODUCTION IN-VIVO AND DE-NOVO GENERATION IN-VITRO

The molecular mechanisms underlying the generation of the various type s of cells in the vertebrate retina are largely unknown. We investigat ed the possibility that genes belonging to the basic helix-loop-helix (bHLH) family of transcriptional factors participate in cell-type spec ification during retinal neurogenesis, Chick neuroD was isolated from an embryonic cDNA library and its deduced amino acid sequence showed 7 5% identity with mouse neuroD, In situ hybridization showed that neuro D was expressed in cells located at the outer portion of the developin g retinal neuroepithelium, the location where prospective photorecepto rs reside. Misexpression of neuroD in retinal neuroepithelium through replication-competent, transformation-deficient retroviruses produced a retina with three, instead of two, layers of photoreceptor cells; th e number of cells that express visinin, a marker for cone photorecepto rs, increased over 50% compared to control embryos misexpressing the g reen fluorescent protein. No significant changes were observed in the number of other retinal neurons, including those that express RA4 (gan glion cells), pax6 (ganglion cells and amacrine cells), and chx10 (bip olar cells). Retroviral-driven misexpression of neuroD in monolayer cu ltures of retinal pigment epithelium yielded de novo production of pho toreceptor cells with no other types of retinal neurons detected. We p ropose that neuroD is important for photoreceptor cell production in t he vertebrate retina. (C) 1998 John Wiley & Sons, Inc.